Suppress spurious error output during LFS push

kart/lfs_commands/__init__.py

@@ -14,7 +14,6 @@
 from kart.object_builder import ObjectBuilder
 from kart.rev_list_objects import rev_list_tile_pointer_files
 from kart.repo import KartRepoState
-from kart.subprocess_util import subprocess_tee
 
 EMPTY_SHA = "0" * 40
 
@@ -99,30 +98,40 @@ def push_lfs_oids(repo, remote_name, lfs_oids):
     push all of those LFS blobs from the local cache to the given remote.
     """
     # Older git-lfs doesn't support stdin so we fall back to using args if we somehow have an older version.
-    if not _push_lfs_oids_using_stdin(repo, remote_name, lfs_oids):
+    if _git_lfs_supports_stdin(repo):
+        _push_lfs_oids_using_stdin(repo, remote_name, lfs_oids)
+    else:
         _push_lfs_oids_using_args(repo, remote_name, lfs_oids)
 
 
+def _git_lfs_supports_stdin(repo):
+    r = subprocess.run(
+        ["git-lfs", "push", "?", "--object-id", "--stdin"],
+        env=tool_environment(),
+        cwd=repo.workdir_path,
+        capture_output=True,
+        text=True,
+    )
+    return "unknown flag: --stdin" not in r.stderr
+
+
 def _push_lfs_oids_using_stdin(repo, remote_name, lfs_oids):
-    # TODO - capture progress reporting and do our own.
     with tempfile.TemporaryFile() as oid_file:
         oid_file.write("\n".join(lfs_oids).encode("utf-8"))
         oid_file.write(b"\n")
         oid_file.seek(0)
 
-        returncode, stdout, stderr = subprocess_tee(
-            ["git-lfs", "push", remote_name, "--object-id", "--stdin"],
-            env=tool_environment(),
-            cwd=repo.workdir_path,
-            stdin=oid_file,
-        )
-        if returncode == 0:
-            return True
-        elif b"unknown flag: --stdin" in stderr:
-            return False
-        else:
+        try:
+            # TODO - capture progress reporting and do our own.
+            subprocess.check_call(
+                ["git-lfs", "push", remote_name, "--object-id", "--stdin"],
+                env=tool_environment(),
+                cwd=repo.workdir_path,
+                stdin=oid_file,
+            )
+        except subprocess.CalledProcessError as e:
             raise SubprocessError(
-                "There was a problem with git-lfs push", exit_code=returncode
+                f"There was a problem with git-lfs push: {e}", called_process_error=e
             )